Mammography is the process of obtaining x-ray images of the human breast for diagnosis or surgery. It involves positioning a patient's breast in a desired orientation against a cassette holder (also known as a “bucky”) of a mammography unit, compressing the breast with a compression device (e.g., a compression paddle), and then exposing the breast to x-rays to create a latent image of the breast on an image receptor. After exposure, the compression device is released. The image receptor is usually a film in contact with an intensifying screen contained within a cassette. The cassette is inserted into a cassette holder before every image is taken and removed after every image. The film is removed from the cassette and developed to produce a radiographic image of the breast.
A complete mammographic study usually involves at least two x-ray exposures of each breast. One exposure is a craniocaudal view in which the breast is compressed in a superior-inferior direction, i.e., from the direction of the patient's head downward, against a tube-side surface of the cassette holder. The plane of the tube-side surface of the cassette holder is parallel to the floor and the x-ray beam is directed vertically downward. A second exposure is a lateral or oblique view in which the breast is compressed mediolaterally, i.e., from the direction of the patient's midline sidewise, against the tube-side surface of the cassette holder which is angled, along with the axis of the x-ray beam, relative to the floor.
The compression device includes a rectangular flat plate, called a compression paddle or a compression plate, which is attached to the mammography unit between an x-ray tube assembly and the cassette holder (also known as a “bucky”). The edges of the paddle or plate are turned upward away from the cassette holder to provide a smooth curved surface for patient comfort. The compression paddle is usually made of thin, light-transparent, plastic that absorbs only a small fraction of the incident x-ray beam. The compression paddle is moved either manually or by power drive to apply a compression force to the breast, thereby flattening the breast against the cassette holder to a near uniform thickness. U.S. Pat. No. 6,049,583 issued to the present inventor discusses methods and apparatus for measuring compression force in mammography. During compressing and imaging, parts of the patient's body come into contact with the compression paddle. After x-ray exposure, the compression force is released for patient comfort. Sometimes, pads that are not light transparent are used in conjunction with compression paddles, but by doing so, the visible light that reaches the tube-side surface is blocked which hinders proper placement of the breast.
To properly position the patient's breast in a desired orientation, a technologist is guided by a light beam originating from the x-ray tube assembly that passes through a collimator and the compression paddle and illuminates the area of the cassette holder that will be exposed to x-rays, i.e., the imaging area. As is well known in the field, to properly position the breast, the patient's chest wall or other regions of the body, depending on the desired view, are brought into tight contact with the rigid surfaces of the cassette holder, its edges, and corners. This procedure has the effect of forcing the patent's anatomy to contour to the shape of the cassette holder, which often causes patient discomfort and pain.
Oftentimes, overlapping internal structures are present within the breast tissue that can obscure their delineation in a radiographic image. As a result, it is often necessary to reposition the breast slightly in order to arrive at a diagnosis. This requires repositioning the patient for each view with the attendant discomfort.
During positioning, compressing, and imaging, parts of the patient's body come into contact with the cassette holder. The cassette holder is a rectangular, box-like device that has a flat tube-side surface against which the breast is compressed, a flat outer surface along one edge of the tube-side surface which contacts the patient's chest wall or torso, and two flat side surfaces opposite each other along the other edges that can come into contact with other parts of the patient's anatomy such as the underarm and axilla. Each of the side surfaces has an opening, typically rectangular, to a cassette tunnel. The openings are used for insertion and removal of the cassette. The tube-side surface includes an imaging area, which is transparent to x-rays, located directly above the cassette as it resides in the cassette holder, and where the breast is positioned during imaging, and a solid section which is not transparent to x-rays. Within the cassette holder is an antiscatter grid assembly. The cassette holder is held in position on the x-ray unit by slidably engaging to a support member. Because the surfaces of the cassette holder may come into contact with blood or other infectious material, they must be able to withstand contact with the chemical agents usually used for disinfecting purposes. Cassette holders come in different sizes depending on the size film to be used.
It is well known that many women find the procedure for obtaining a mammogram to be uncomfortable and for some, even painful. Methods to provide patient comfort during the examination involve adding cushioning material to the surfaces of the cassette holder and/or the compression paddle.
It is well known to those in the art that image quality of a mammogram is highly dependent on beam quality, which is a function of several factors including the kilovoltage (kVp) impressed across the anode and cathode of the x-ray tube, the material of the x-ray target (e.g., molybdenum), the inherent filtration of the tube (e.g., beryllium), and the material and thickness of added filtration (e.g., molybdenum). Beam quality is measured in terms of half value layer in aluminum (HVL). Adding material in the path of the x-ray beam has a similar effect to adding filtration, for example, HVL increases. Most modern mammography units automatically adjust x-ray exposure factors, including kVp, according to HVL. Increasing kVp decreases image contrast, and thus reduces image quality.
The degree to which HVL is increased by the addition of a material in the path of the x-ray beam depends on its linear attenuation coefficient, “μ,” and thickness, “t,”. Linear attenuation coefficient is related to physical density. For example, material described as being made of foam can have a density that varies over a wide range. For example, polyurethane foam can have densities of between about 1.8 and about 2.6 pounds per cubic foot.
Moreover, foam cushioning comes in various thicknesses. The firmness of foam is measured in units of Indentation Force Deflection (IFD). which is determined by indenting (compressing) a foam product 25% of its original height. The amount of force, measured in pounds, required to compress the foam is its 25% measurement. The IFD of cushioning foam can range from between about 21 and about 45 pounds.
With respect to the thickness of the foam, it is known in the art that the sharpness of a radiographic image depends on the object to film distance. The shorter the distance the sharper the image. This is an important consideration when attempting to identify images of breast calcifications which are probably the most important diagnostic indicator of early breast cancer. Interposing cushioning material between the breast and the surface of the cassette holder increases the distance between the calcifications and the film and decreases the sharpness of their images.
Since different kinds of foam can be supplied with varying thicknesses and firmnesses, the application of equal compression force to different cushioning materials can drastically impact arriving at a proper diagnosis. Without intending to be bound by theory, this is likely caused by different distances between the calcifications and the film depending on the cushioning material used which result in different degrees of image sharpness. Knowing exactly what outside factors have impacted a mammogram can be an important consideration in arriving at a correct diagnosis.
Subject to a woman's family health history, women are encouraged to obtain their first mammogram at around age 40 and annually thereafter. In reading and analyzing mammograms, images of a current examination are compared with previous examinations. A radiologist or other medical professional looks for the appearance of and/or changes in diagnostic markers such as micro-calcifications and other internal structures. The difficulty in reading mammograms is that changes in these images can be very subtle and depend in large measure on image quality. Other factors that affect image quality are motion of the breast during x-ray transmission and the range of optical contrast in the image. Motion of the breast is inversely affected by the magnitude of compression force applied by the compression paddle, a larger force reduces movement of the breast. If less compression force could be used, however, then the examination would be less uncomfortable for the patient.
The range of optical contrast in the image depends on several factors including the level of x-ray exposure. For patients with dense breasts, the peripheral region of the breast is often too dark, making it difficult to identify diagnostic features in this area. The usual method to improve visibility in this region is to bright light the film, i.e. increase the light intensity behind the film, but this can reduce contrast perception. Other methods to overcome this problem have been suggested including the use of different kinds of x-ray attenuators positioned between the compression paddle and the breast or under the breast, e.g. a water bag or a solid, elastic, unit density plastic, but these devices are sometimes difficult to implement. Other methods have been described including, for example, the use of different kinds and shapes of x-ray filters positioned between the x-ray tube and image receptor and adjusting the quality and/or the intensity of the x-ray beam using sensors and computers to determine the attenuation and shape of the breast. For a variety of reasons none of these other methods are in general use.
Since a wide variety of cushioning materials may be used in mammography, including some that can have a negative effect on image quality, there is a need for a viewer, e.g., a technologist or a radiologist or other medical professional, be able to determine from the mammogram, for example, that cushioning material was used and the type of cushioning. There is also a need that the mammogram contain a permanent record regarding the nature of the cushion material used.
Further, there remains a great need for comfort devices for use during mammography which can minimize or eliminate the pain and discomfort experienced by the patient. To be useful in clinical practice such devices must also not add significantly to the cost of the examination. There remains a great need to provide an indication on a mammogram to alert the viewer that the quality of the breast image may have been compromised by the use of a comfort device. There also exists a need to provides information to a medical professional, e.g. a technologist or radiologist, regarding the use of comfort devices without compromising the cushioning effect of the materials. In addition, there is a need for comfort materials for compression devices that still allow visible light to be transmitted to allow for proper positioning of a breast. There is a need for a device and method to limit motion of the breast without excessive compression force. And there remains a need for a quick, simple, inexpensive way to improve the visibility of the peripheral region of the breast on mammograms.